1. Field of the Invention
The invention relates to apparatus and methods for handling cryogenic fluids and in particular to apparatus and methods for conditioning liquefied natural gas for fueling motor vehicles.
2. Description of Related Technology
Cryogenic fuel tanks employed to contain highly volatile fuels, such as liquefied natural gas, typically store such fuels at low temperatures and pressures. Temperatures of about -252.degree. F. to about -186.degree. F. and pressures of about 5 psig to about 150 psig are typical. Low pressure storage is preferred, for example, because the trailers for delivery of liquefied natural gas also have a low design pressure in order to keep the weight of the trailers at a minimum. Also, customers typically prefer low pressure liquefied natural gas delivery to provide them with a maximum storage time before pressure rise in the storage tank requires venting.
Vehicle fuel systems, on the other hand, may require pressures in the range of at least 100 psig for satisfactory operation of the vehicle engine. Dispensing liquefied natural gas from a low pressure tank to a vehicle tank is therefore problematic. One problem is that if a low pressure liquid is pumped or otherwise transferred to a vehicle tank, and the pressure is raised inside the vehicle tank by adding vapor to the tank, vehicle motion can mix the cold liquid with the pressurizing vapor to condense away the pressure. Thus, the vehicle engine is supplied with fuel at a pressure insufficient for open throttle operation.
Gustafson, U.S. Pat. No. 5,228,295 and Kooy et al., U.S. Pat. No. 5,325,894 both teach the addition of vapor from a vapor space of a liquefied cryogenic fuel storage tank to a delivery conduit through which liquefied cryogenic fuel flows to a vehicle tank. The Gustafson patent, however, teaches flowing only a small amount of vapor from a storage tank to a delivery conduit in order to maintain pressure in the storage tank below a predetermined maximum pressure. There is no disclosure in the Gustafson patent of utilizing this small inflow of vapor to modify the equilibrium pressure of the liquefied cryogenic fuel. Furthermore, the apparatus disclosed in the Gustafson patent appears to lack the ability to fuel against a pressure much higher than the pressure of the fuel storage tank. A vehicle tank having a pressure of only a few pounds per square inch higher than the fuel storage tank would appear to cause the pump discharge to back flow to the top of the fuel storage tank rather than flow to the vehicle tank.
The Kooy et al. patent discloses flowing vapor from a liquefied cryogenic fuel storage tank, through a compressor, and directly into a liquefied cryogenic fuel delivery conduit. With reference to FIG. 3 of Kooy et al., a control valve disposed in the delivery conduit acts as a direct contact heat exchanger. A temperature controller disposed downstream of the valve regulates the control valve and thus the amount of liquefied cryogenic fuel flowing through the control valve relative to the amount of vapor supplied to the control valve. Problems with such a system may include the lack of control. A control valve alone cannot control the pump and compressor discharges because both the temperature and amount of flow of such discharges may vary. In the extreme case, neither the pump nor the compressor can run if its discharge flow must be stopped. Furthermore, in an apparatus according to the Kooy et al. patent, the response time of the mechanical equipment would most likely be too slow to keep up with the flow of a fueling operation. Also, the compressor discharge gas could quickly separate from the pump discharge liquid, greatly reducing heat transfer. Once the gas enters the vehicle fuel tank, the gas may only add head pressure and not change the liquid's equilibrium condition until the vehicle leaves the fueling station. Then the fuel tank pressure would drop as the colder liquid condenses the head space vapor and full throttle performance would not be possible.
Another problem that may occur in an apparatus disclosed in the Kooy et al. patent involves the size of compressor required for the conditioning operation. In such an apparatus, the compressor would need to be large enough to provide vapor at the maximum surge rate resulting in increased cost of oversizing from the normal flow requirements.